"It has become clear that no consensus has developed to support the administration's [bailout] proposal," said McCain in a statement. "I do not believe that the plan on the table will pass as it currently stands, and we are running out of time."

In political terms, this move could be a huge risk for the GOP nominee. He is thrusting himself into the center of an issue on which he has struggled to explain himself to voters. It only emphasizes that the economy is far and away the No. 1 issue in the election – and voters generally say that they trust Democrats more than Republicans to handle the economy well.

It's also possible, though, that McCain's move will be seen by voters as an action undertaken by a forceful leader in a crisis. A Pew Research Center poll shows that Americans back the administration's $700 billion bailout plan by a 2-to-1 margin (although a Rasmussen Reports telephone survey showed a more even split).

In response to McCain's unilateral move, Senator Obama said that he stood ready to go to Washington to help push through the bailout process, if leaders thought it would be useful, but he added that it was more important than ever to debate the economic problems at issue. Obama said he had initiated a plan earlier in the day that the two campaigns issue a joint statement on points they agreed were necessary in any financial bailout, including the need for limits on the pay of top executives whose companies would sell troubled securities to the government.

"The university has gone all out for this and the town has gone all out," said one debate official, who was not authorized to speak to the press . "It will be an absolute tragedy if they call it off now."

In Washington, Senate majority leader Harry Reid also criticized the idea of candidates trying to move the process forward. "It would not be helpful at this time to have them come back during these negotiations and risk injecting presidential politics into this process," he said in a statement Wednesday. "We need leadership; not a campaign photo op."

President Bush is set to address the nation on Wednesday evening on the bailout's importance.

If McCain, Obama, and Mr. Bush agree to some sort of joint front or conference pushing aid for the financial system, it could mark a crucial push for the plan. At the least, the move on the debates marks another twist in a campaign that has been the most unpredictable in a generation.